For several decades, it has been a goal of industry to make plastic sheet or film materials either environmentally degradable by sunlight, moisture, temperature and the like or biodegradable by microorganisms. Usually after environmental degradation, plastic sheet or film materials are then more susceptible to assimilation by microorganisms. In spite of considerable efforts, our lands are becoming inundated with plastic sheet or film materials, and articles made therefrom, that will not degrade perhaps for centuries. It is therefore a continuing goal to make plastic sheet or film materials as fully degradable as possible. A biodegradable material is one that undergoes biological degradation which ultimately mineralizes (biodegrades to CO.sub.2, water and biomass) in the environment like other known biodegradable matter such as paper and yard waste. It would be highly desirable to provide a plastic sheet or film material that is biodegradable especially in a municipal solid waste facility where it may undergo biodegradation in the presence of heat, moisture and microorganisms.
There is a particular need for biodegradable plastic sheet or film material in disposable diapers, sanitary pads, hygienic pads and the like. These products for practical purposes must satisfy such properties as water impermeability in order to prevent seepage of urine and other human waste products therethrough. In addition, such sheet or film materials must have sufficient tear, tensile and impact strengths to function in such useful articles. The same properties that make them useful, however, lead to their lack of biodegradability. A few examples of patents directed to biodegradable and environmentally degradable compositions or products include U.S. Pat. Nos. 3,901,838; 3,921,333; 4,016,117; 4,021,388; 4,120,576; 4,125,495; 4,218,350 and 4,420,576.
A number of problems exist in connection with certain biodegradable films. For instance, biodegradable thermoplastic starch-based polymeric films are known as disclosed in International Applns. Nos. EP90/01286 and WO91/02025. However, these films tend to be dry, brittle and moisture sensitive. Over time such films absorb water causing them to become soggy and they eventually disintegrate. Although biodegradable, their moisture sensitivity renders them unsuitable for use as moisture barriers in practical applications. Other thermoplastic water impermeable films have been proposed. But such films are usually tough and stiff thereby rendering them uncomfortable for use with diapers, sanitary pads, hygienic pads and the like. Therefore, against this background of prior art, it is evident that further improvements in biodegradable films are needed.